The present invention relates to oxymethylene polymer molding compositions. More particularly, the present invention relates to glass-reinforced oxymethylene polymer molding compositions that contain a minor amount of a prepolymeric methylol amino resin, and which can be molded into articles having enhanced mechanical properties, particularly enhanced impact strength.
Oxymethylene polymers having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde.
The usefulness of oxymethylene polymers in molding compositions has also been known for years. For example, U.S. Pat. No. 3,275,604 discloses a moldable oxymethylene copolymer comprising oxymethylene groups and oxyethylene groups, with said oxyethylene groups having a --CH.sub.2 OR group pendant therefrom, which may be prepared by polymerizing a source of oxymethylene units such as trioxane with a glycidyl ether.
Various additives and improvements to oxymethylene molding compositions have been developed. For example, to permit molding objects of such compositions at relatively high temperatures without adversely affecting the surface characteristics of the molded objects, thermal stabilizers have been included in minor proportions. Compounds used as thermal stabilizers have included amine-substituted triazine monomers (U.S. Pat. No. 4,342,680), hydroxymethylol cyanoguanamine (U.S. Pat. No. 3,626,024), and cross-linked melamine-formaldehyde polycondensates (U.S. Pat. Nos. 4,139,575 and 4,230,606). U.S. Pat. No. 4,386,178 discloses polyacetal or oxymethylene polymer molding materials containing as a thermal stabilizer from 0.1 to 10 weight percent of a melamine resin described as a melamine-formaldehyde condensate having a mean degree of polymerization of from 1.2 to 6.0 and a melamine:formaldehyde ratio of from 1:2 to 1:5.5, which has been partially etherified with a C.sub.1 -C.sub.4 alkanol so that the ratio of melamine:ether radicals is from 1:1.5 to 1:5.0 and the ratio of melamine:free methylol groups is from 1:0.5 to 1:3. The molding compositions can contain various conventional additives, including fillers of, e.g., glass fibers. The examples disclose increases in the melt index of the compositions containing the stabilizer, but no tests of mechanical properties of objects molded therefrom.
Numerous efforts have been made to improve the mechanical properties of objects molded from oxymethylene polymer molding compositions containing fibrous reinforcing agents such as glass fibers. For example, U.S. Pat. No. 3,963,668 discloses thermoplastic molding compositions of polyoxymethylenes containing 10-50 weight percent glass fibers and 0.1-5.0 weight percent of an alkoxymethyl urea, the inclusion of which is said to improve the tensile strength, flexural strength and impact resistance of molded objects. Conventional additives such as thermal stabilizers can be included. German Patent Specification DE-OS No. 2,921,167 discloses molding compositions of polyoxymethylenes containing 10-50 weight percent glass fibers and from 0.1 to 3 weight percent of an alkoxymethylmelamine monomer containing an average of at least two alkoxymethyl groups. This additive is said to improve the tensile, bending and impact strengths of objects molded from the resulting compositions, without discoloration in the molding process.
Despite the various efforts to improve the properties of reinforced oxymethylene polymer molding compositions, there is further room for improvement in these materials, particularly as to the mechanical properties of objects molded from thermally stabilized molding compositions.
Accordingly, it is an object of the present invention to provide an oxymethylene polymer molding composition that can be molded into objects which exhibit enhanced mechanical properties.
Another object of the present invention is to provide an improved glass fiber-reinforced oxymethylene polymer molding composition that can be molded into articles which exhibit enhanced mechanical properties such as tensile and flexural properties and impact strength.
Still another object of the present invention is to provide a glass fiber-reinforced oxymethylene polymer molding composition that is thermally stable at useful molding temperatures.
These and other objects, as well as the scope, nature and utilization of the present invention, will be apparent to those skilled in the art from the following description, the drawing, and appended claims.